Vertical Dynamics (Suspension)
Suspension - Demands and Possibilities of Implementation
Roads commonly used by motor vehicles are uneven. This unevenness induces vertical displacements of the vehicle and the passengers in the process of driving.
The vehicle comes into contact with the road over the tire. Road unevenness which is negligible compared to the size of the tire contact patch can be compensated by tire elasticity, whereas larger unevenness entails a vertical acceleration or deflection of the wheels. In order not to transfer these accelerations to the vehicle body, a displacement compensating element has to be placed between the wheel and the vehicle body.
Steel springs are technically the simplest displacement compensating elements. As a result, they are also the most commonly used displacement compensating element, where the spring force is a function of displacement. It is commonly used in the suspensions of motor vehicles. An oscillatory system results when various elements are connected together over springs. Hence an additional energy absorbing element, the damper, has to be included.
The objective of the suspension in the motor vehicle is to reduce these vertical movements. The essential criteria defining the quality of a suspension can be listed as follows:
Suspension comfort for the passengers (Effective acceleration affecting the passengers)
Forces affecting the load (Effective value of structure acceleration)
Wheel load fluctuations (Effective value of the dynamic wheel load), which influence the grip between tires and road (driving safety) and the transferable load on the road surface.
A number of further demands which are partially contradictory, are made on the suspension of a motor vehicle (see figure).
Before dealing with the technical details of the spring and damper elements, the road and the mathematical description of its unevenness is firstly presented.